FIELD OF INVENTION:
Disc type power semiconductor thyristors are widely in use for various industrial
applications. The configuration and mounting method of the thyristor assembly depends
on specific industrial application. The assembly may contain a single or multiple thyristors
mounted on a single or a multiple heat sink or other typical configurations customized for
specific industrial application. The present invention generally relates to such thyristors
assembly and specifically relates to an assembly of two thyristors mounted on a common
heat sink for rotating electrical machinery used for power generation.
BACKGROUND:
Disc-type semiconductor thyristors are based on compression bonded technology,
whereby the internal components of the thyristor including the silicon chip are stacked
one over the other and the contacts between them are established through an externally
applied compressive force. Disc-type thyristors, therefore, require use of appropriate
spring clamps for mounting on heat sinks. The exact configuration of the complete
thyristors assembly with heat sinks and spring clamps is determined by the specific
application requirement. Patent documents US Patent Nos. 5168425, 3955122, 4128810,
4224663, 4766653 describe various cases of prior art, whereby an assembly contains a
single or multiple thyristors with a single or multiple heat sinks to match the requirements
of the electrical circuit configuration. An AC controller circuit for welding applications
employs two thyristors connected in anti-parallel arrangement, both of which are
sandwiched between two water cooled heat sinks. Similarly, high voltage DC transmission
circuits employ as many thyristors in series as required to support the system voltage and
water cooled heat sinks between two successive thyristors.

Invariably, in all the cases, mounting of the thyristors on the heat sinks with the specified
clamping force require special tooling and the exact methodology and arrangement
depend on specific construction of the assembly.
As in the cases of prior art the present invention too is related to a unique custom-
specific requirement. The feature that distinguishes the present invention from the prior
art is that the assembly is especially conceptualized and constructed to facilitate
mounting on the rotating wheel of an electrical machinery used for power generation.
The assembly has several novel features particularly in respect of the heat sink
construction, insulating enclosure, mounting spring and layout of gate leads.
The object of the invention is to construct an assembly that contains two disc type power
semiconductor thyristors electrically connected in series and mounted on a common heat
sink.
The invention has the object to construct the heat sink with an arrangement of fins that
facilitates air circulation around the thyristors and with suitable provisions to guide the
gate leads of the thyristors.
The invention has also an object to design a spring clamp system to mount the thyristor
assembly on the heat sink with a specified load.
The invention has yet another object to design an insulating enclosure to house the
thyristor, spring clamp and other interfacing components to provide the required isolation
between the AC and DC terminals of the assembly.

Another object of the invention is to formulate a method of mounting the two thyristor
assembles on the heat sink and to construct a set-up for fabricating the assembly in
industrial manufacturing.
SUMMARY OF INVENTION:
The invention employs two disc-type thyristors in a single heat sink package that serves
as a half-bridge assembly (one phase) of three-phase controlled rectifier. Both the
thyristors (rating 1800V/800A) are mounted on a common air cooled heat sink using
12kN spring clamps.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Fig. 1- shows 3-phase bridge rectifier circuit where the assembly is used.
Fig. 2- shows outline diagram of disc-type power semiconductor thyristor.
Fig. 3- shows front view of the two thyristor assemblies in series.
Fig. 4- shows top view of heat sink assembly of two thyristors in series.
Fig. 5- shows a sketch of arrangement of heat sink fins.
Fig. 6- shows side view of heat sink assembly showing one of the two thyristors.
Fig. 7- shows a sketch of electro-pneumatic arrangement for mounting the thyristors.
Table 1- shows measurement of reverse and forward off-state voltage.
Table 2- shows measurement of gate-to-cathode resistance.

DETAILED DESCRIPTION OF ACCOMPANYING DRAWINGS:
The invention will now be described in an exemplary embodiment as depicted in the
accompanying drawing. There can however be other embodiments of the same invention,
all of which are deemed covered by this description.
The invention is related to construction of an assembly of two disc-type semiconductor
thyristors (1) that are connected electrically in series. In the application, the assembly, as
a single entity, represents one phase of the fully controlled bridge rectifier, as shown in
Fig.l. The joint between the two thyristors (2) represents the AC input, while the
unconnected cathode (3) and anode ends (4) of the assembly form the DC terminals.
Fig.2 shows the outline diagram of a typical disc-type thyristor.
The assembly employs a single heat sink on which both the thyristors are mounted. The
heat sink (5) represents the AC input terminal of the assembly. For the purpose of
fulfilling the electrical circuit representation as in Fig-1, both the thyristors are placed in
vertically opposite direction to each other as shown in Fig-3. Thus, the heat sink
represents anode polarity for one thyristor and cathode for the other. The electrodes (3,
4) which are made up of conventional high-strength copper-chromium-zirconium alloy
and placed on the other sides of the thyristors, represent the DC terminals. As shown in
Fig.4, the insulated gate leads (6) of the thyristors are drawn sideways from the
assembly through horizontal tunnel-like conduits within the heat sink. The orientation of
the two leads is arranged symmetrically with reference to the -diagonal of the horizontal
plane of the heat sink. The lead ends (7) are crimped with insulated ring-tongue
terminals for making electrical connection to the gate circuit of the bridge rectifier at the
user end.

Suitable cable glands (8) are provided to hold the leads firmly to the heat sink. The
threaded glands are fixed to the heat sink with application of glue, in a conventional
manner followed for rotational applications, to ensure that the connection is rigid and
intact during rotation. The thyristor together with the copper electrode, disc-spring (9)
and steel plate are enclosed within a rigid plastic enclosure (10). A steel plate (11) is
placed over the top of the enclosure and the whole arrangement is fastened to the heat
sink using four bolts (12).
The heat sink is made up of a conventional aluminum alloy that is used for rotating
application. The heat sink is provided with four mounting holes to facilitate fixing of the
assembly on the rotating machinery at the user end. The pitch and size of the mounting
holes are selected based on application requirement. The fins (14) of the heat sink are
arranged in five rows on either side (Fig.5). The centre portions of the rows (15) closer to
the thyristors are provided with relatively lower elevation to facilitate better air circulation
and to minimize heat trapping near and around the thyristors. The overall surface area of
the fins is maintained at a specified value based on empirical data. The heat sink is
provided with tunnel-like conduits (16) to guide the gate leads from the thyristor gate
terminals to the cable gland outlets. The heat sink is provided with an M12 threaded hole
(13) to facilitate electrical bus-bar connection at the user end.
The disc-type thyristor related to the present invention requires an external mounting
force of 12kN in order to achieve the electrical and thermal contacts between the
semiconductor chip and the interfacing components that form part of its internal
structure. This design requirement is met through appropriate selection of belleville type
disc-spring (9) (Fig-6) made up of spring steel alloy 50CrV4 that is conventionally used
for semiconductor devices in both static and rotational applications.

One spring of 60 mm outer diameter and 3 mm thickness is used per thyristor and the
spring offers 12kN load at a deflection of 1.30 mm, which is achieved by suitably
designing the build-up of the components in the assembly.
The thyristor has a peak-inverse-voltage rating of 1800V that comes across its cathode
and anode terminals. The material and dimensions of the plastic enclosure that is used to
house the thyristor are selected appropriately to meet the electrical isolation between the
two polarities.
The enclosure also requires adequate compressive strength to withstand the mounting
load without any deformation. For this purpose, it is designed using glass-filled polyester
material that offers excellent dielectric (12 KV/mm) and compressive strengths (1800
Kg/cm2) and that has proven experience data in rotational applications. Steel plates of
2.5 mm thickness are additionally employed on either side of the roof of the enclosure to
provide mechanical reinforcement to the insulation material. The enclosure is provided
with an undulated structure with a creep distance of 29 mm for electrical isolation
between the steel-plate and the copper electrode, which have opposite polarities (anode
and cathode). This compares well with the conventional creep paths of around 22 mm
adopted for a 2500V thyristor and 30 mm for a 4500V thyristor.
The enclosure is also provided with an internal recess to accommodate the gate terminal
protruding away from the circular periphery of the thyristor. The wall thickness of the
enclosure is selected appropriately to withstand the applied load. The selection of internal
height determines the gap between the enclosure and the heat sink before applying the
load. As this gap is nothing but the deflection of the spring in the fully mounted
condition, it is maintained at 1.30 mm to achieve 12kN load.

Four numbers of M6 size bolt are used to fasten the thyristor assembly to the heat sink.
The length of the bolt that gets threaded into the heat sink is appropriately selected to
withstand the mounting load so that the steel bolt does not cause any strain or damage
to the softer aluminum heat sink. A threaded depth of 17 mm is chosen on an empirical
basis.
The gate terminal of the thyristor is soldered to an insulated copper lead. The lead is
then bent at 90° angle near the terminal end in opposite directions for the two thyristors.
The soldered region is then reinforced with two layers of polyoiefin heat-shrinkable
insulating sleeves that are shrunk one over the other. The reinforcement provides
strength to the soldered joint and integrity of the lead together with the terminal. The
overall length of the lead is selected based on user requirement.
The thyristor assembly is mounted using a pneumatically operated arrangement, which
consists of a piston (20) that moves in the downward direction when the air cylinder (18)
is activated. The cylinder is fixed on a rigid structure supported by guiding pillars. An
electrically operated solenoid valve is used to move the piston up and down. An
alignment fixture is used to position the heat sink assembly. Following are the steps
adopted in the process of mounting.
The heat sink assembly with all the components assembled is loaded on the alignment
fixture (19). A steel cup (17) is placed in an inverted manner on the steel plate at the top
of one of the two thyristor assemblies. The purpose of the cup is to clear the projecting
DC terminal so that the piston does not apply load on the terminal.

The set-up is switched ON. The piston moves down and presses the steel cup. The gap
between the enclosure and the heat sink gets closed, as the piston presses down the
assembly. In the activated position, the bolts are individually tightened. The set-up is
then switched OFF and the piston is released upwards. The position of alignment fixture
(19) is adjusted for mounting the second thyristor assembly.
Prototypes of thyristor assemblies were fabricated and the following checks were carried
out.
a) Reverse and forward off-state blocking voltage of both the thyristors in the
assembly were measured and compared with the measurements taken on the
thyristors before the assembly. As tabulated in Table-1, the values taken at 60 mA
of leakage current are found to be comparable. As no significant differences are
observed, it is confirmed that the undulated creep path on the plastic enclosure is
adequate to realize the required electrical isolation.
b) The resistance between gate and cathode terminals of the thyristors mounted on
the prototype assemblies was measured and the values were found to be
comparable with the measurements taken before assembling (Table-2).
c) The prototypes were subject to a load of up to 15 kN and no visible deformations
were observed on the plastic enclosure, steel plates and DC terminal electrodes.
d) The load-deflection characteristics of the disc spring was checked and found to be
within the desired design limits.

e) The heat sinks were removed from the fully fabricated prototypes 48 hours after
the assembly and the M6 threaded holes on the sink were examined for any
possible damages due to bolting and tightening. The threads were found to be
intact. When the prototypes were again assembled using the same heat sinks,
there was no strain observed while fastening the bolts.

WE CLAIM:
1. A device mountable on a rotating electrical machinery for power generation,
comprising :-
• thyristor assembly (1)
• heat sink (5)
• disc spring (9)
• insulating enclosure (10)
• gate leads (6)
• electrodes (3, 4)
• steel plates
Characterized in that two thyristors are mounted on a common heat sink.
2. The device as claimed in claim 1, wherein thyristor assembly (1) consists of two
1800V/800A disc type power semiconductor thyristors' electrically connected in
series configured in vertically opposite direction to each other.
3. The device as claimed in claim 2, wherein the electrodes (3, 4) are made of
conventional high strength copper-chromium-zirconium alloy and are placed on
either side of the thyristors, and the unconnected cathode (3) and anode (4) ends
of the assembly forms the DC terminals.

4. The device as claimed in claim 2, wherein the assembly of two thyristors, as single
entity, represents half bridge assembly of one phase of the fully controlled three
phase bridge rectifier.
5. The device as claimed in claim 2, wherein the insulated gate terminal of the
thyristor is drawn side ways from the assembly through horizontal tunnel like
conduit (16) within the heat sink (5).
6. The device as claimed in claim 5, wherein orientation of the two gate leads (6) is
arranged symmetrically with reference to the diagonal of the horizontal plane of
the heat sink assembly (5).
7. The device as claimed in claim 6, wherein the lead ends are crimped with insulated
ring-tongue terminals.
8. The device as claimed in claim 7, wherein suitable cable glands (8) are provided to
hold the leads firmly to the heat sink and the threaded glands are fixed to the heat
sink with conventional glue used for rotational application.
9. The device as claimed in claim 1 wherein the thyristors (1) together with copper
electrode (3, 4), disc spring (9) and steel plate are enclosed within a rigid plastic
enclosure (10) and a steel plate is placed over the top of the enclosure and the
whole arrangement is fastened to the heat sink (5) using four bolts (12).

10. The device as claimed in claim 1, wherein the heat sink (5) is made up of an
aluminum alloy suitable for rotational application.
ll.The device as claimed in claim 10, wherein the heat sink is provided with a M12
hole (13) for connecting A.C terminals.
12. The device as claimed in claim 10, wherein the fins (14) of the heat sink are
arranged in five rows, and the centre portion of the rows closer to the thyristors
(15) are provided with lower elevation.
13.The device as claimed in claim 1, wherein a belleville type disc-spring (9) made up
of spring steel alloy 50 cr V4 suitable for rotational application is used.
14. The device as claimed in claim 13, wherein a spring of 60 mm diameter and 3 mm
thickness is used per thyristor and the spring offers 12 kN load at a deflection of
1.30 mm.
15. The device as claimed in claim 1, wherein the plastic enclosure made of glass filled
polyester material have a compressive strength of 1800 kg/cm2 and dielectric
strength of 12kV/mm.
16. The device as claimed in claim 1, wherein mechanical reinforcement to the
insulation material is provided by 2.5 mm thickness steel plate on either side of the
roof of the enclosure.

17. The device as claimed in claim 15, wherein the enclosure is provided with
additional means to provide electrical isolation between the steel plate and copper
electrode with an undulated structure of the enclosure.
18. The device as claimed in claim 15, wherein an internal recess (21) is provided to
accommodate the gate terminal protruding from away from the circular periphery
of the thyristors.
19. The device as claimed in claim 1, wherein gate terminal of the thyristor is soldered
to an insulated copper lead and the lead is then bent at 90° angle near the
terminal end in opposite direction for the two thyristors and the soldered region is
then reinforced with two layers of polyolefin heat-shrinkable insulating sleeves that
are shrunk one over the other.
20. A system as claimed in claim 1 having pneumatically operated arrangement for
mounting the thyristor assembly comprising:-

• air cylinder (18)
• piston (20)
• rigid structure (21)
• alignment fixture (19)
• solenoid valve
• steel cup (17)
• steel plate

21. The system as claimed in claim 20, wherein pneumatically operated arrangement
consists of a piston (20) that moves in the downward direction when the air
cylinder (18) is activated.
22. The system as claimed in claim 20, wherein the air cylinder (18) is fixed on a rigid
structure (21) supported by guiding pillars.
23. The system as claimed in claim 20, wherein an electrically operated solenoid valve
moves the piston (20) up and down.
24. The system as claimed in claim 20, wherein an alignment fixture (19) is used to
position the heat sink assembly (5).
25. The system as claimed in claim 20, wherein the steel cup (17) is placed in an
inverted manner on the steel plate at the top of one of the two thyristor assembly.
26. A method for mounting the thyristor assembly as claimed in claim 1 consisting step
of:-

• loading the heat sink assembly (5) on alignment fixture (19),
• positioning alignment fixture for mounting one of the two thyristors
assembly,
• placing a steel cup (17) in an inverted manner on the steel plate at the
top of one of the two thyristors assembly,

• switching on the pneumatically operated arrangement,
• tightening the bolts in the activated position,
• switching off the set up,
• adjusting the position of the alignment fixture (19) for mounting the
second thyristor assembly.
27. A method as claimed in claim 26, wherein on switching the pneumatically operated
arrangement as claimed in claim 20 the piston (20) moves down and presses the
inverted steel cup (17) and the gap between the enclosure (10) and heat sink (5)
gets closed as the piston presses down the assembly and in such a position the
bolts are individually tightened and arrangement switched off causing piston to
move upwards and again position of alignment fixture (19) is adjusted for
mounting the second thyristor assembly.

A thyristor assembly (1) of two disc type thyristors mountable on common heat sink (5)
and the entire assembly mountable on a rotating electrical machinery for power
generation. The assembly represents one phase of a 3-phase fully controlled bridge
rectifier circuit in the end application. The heat sink acts as AC terminal and the open
ends of the series connected thyristors act as the two polarities of DC terminals. The
invention employs thyristor of rating 1800V/800A with 14-mm thickness and 32-mm
contact diameter, requiring a mounting load of 12kN. Also invented are a suitable method
to fabricate the complete thyristors assembly and a specific elctro-pneumatic
arrangement to implement the mounting method in industrial manufacturing. Prototype
thyristor assemblies were successfully fabricated using the invented constructions and
methods.